KIEV/MARIUPOL, Ukraine May 25 (Reuters) - Ukrainians were set to give a resounding endorsement to the overthrow of their last elected leader by voting on Sunday for presidential candidates promising close ties with the West in defiance of Russia's Vladimir Putin.

But the absence of over 15 percent of the electorate, in Russian-annexed Crimea and two eastern regions where fighting with pro-Moscow rebels continued on Saturday, may mar any result - and leave the Kremlin questioning the victor's legitimacy, for all President Putin's new pledge to respect the people's will.

Voting began in most of Ukraine at 8 a.m. (0500 GMT) and will end 12 hours later, when exit polls will indicate a result ahead of an official outcome on Monday. But many voters in the Russian-speaking east will find polling stations shut.

European election monitors largely pulled out of Donetsk region for their own safety, citing a campaign of "terror" by pro-Russian separatists against Ukrainian electoral officials.

Polls make a billionaire confectionery magnate known as the "chocolate king" overwhelming favourite, with a high turnout expected on a warm day. The biggest question is whether Petro Poroshenko can take over 50 percent to win in a single round.

He was a strong backer of the protests against Moscow-backed president Viktor Yanukovich last winter and has sought a quick victory by warning that new unrest might prevent a second round.

His closest, if distant, rival is Yulia Tymoshenko, a former prime minister. She seems best placed to contest a runoff in three weeks, but remains a divisive figure to many, more closely linked with the economic failures and corruption that have blighted 23 years of independence.

As Yanukovich's fiercest rival, she may benefit from the fact that few of the 5 million voters in his eastern power base regions of Donetsk and Luhansk may be able to cast ballots for any of the 21 candidates.

Officials say many polling stations will not open for fear of attack and only early on Sunday will they try to distribute ballot papers to those areas where voting may be possible.

Interim Prime Minister Arseny Yatseniuk said on Saturday the act of voting would be victory, hand the new president a mandate to forge closer ties with the European Union and move Ukraine away from a "grey zone of lawlessness and dark forces that dream of suffocating us and into ... a place where it is easier to breathe".

PUTIN TO "RESPECT" RESULT

Western states backed those who took power when Yanukovich fled to Russia three months ago after street protests triggered by his rejection of a free trade pact with the EU. They hope that a mandate for a new leader can help resolve a confrontation with Russia that has sparked military buildups east and west of Ukraine and raised fears of a new Cold War.

Putin pledged on Saturday to "respect" the people's choice and work with Ukraine's new administration - a conciliatory move during an economic forum at which he had acknowledged that U.S. and EU sanctions over Ukraine were hurting the Russian economy.

But he defended his annexation of Crimea in March as a response to the democratic will of the majority ethnic Russian population there. Kiev and its Western allies accuse Moscow of a propaganda war to sow fear among Russian-speakers in eastern and southern Ukraine of "fascist" Ukrainian nationalists and of supporting rebel forces who have seized many towns in the east.

Two weeks ago, separatists in Donetsk and Luhansk regions ran referendums they said let them break from Kiev and opened a way to possibly following Crimea into union with Russia - though Moscow denies any plan to seize any more Ukrainian territory.

Opinion polls before the last few months of violence showed disillusion with Kiev's politicians in the Russian-speaking, industrial east but limited appetite for outright secession.

Putin played down talk of a return to Cold War with the West and dismissed the idea he was bent on restoring the former USSR, whose collapse he has in the past lamented.

Washington and its EU allies are concerned that while Russia may accept the election result, it may use influence in eastern Ukraine to undermine the new president's authority and keep the country beholden to Moscow. Russian officials have questioned the value of holding the vote when the east is in "civil war".

NEW BEGINNING?

A territory on a par with France and with 45 million people, Ukraine is the second most populous ex-Soviet state and plays a pivotal role in relations between Russia and the EU.

Large volumes of Russian natural gas flow across it to Germany and other consumers, creating mutual dependencies that complicate diplomatic calculations on all sides of the conflict.

The inheritor of a patchwork of regions ruled not only from Moscow but by Poland, Austria and others, Ukraine's mix of Russian and Ukrainian speakers as well as ethnic minorities have struggled to forge a common national purpose. But polls consistently show a majority in favour of independence.

Since the 2004 "Orange Revolution" that kept Yanukovich from power, Ukrainians of all stripes have been disappointed with a decade of economic drift and graft that won them the dubious distinction of being named Europe's most corrupt country. Their hopes are pinned on Sunday's vote to start history afresh.

Few of the leading candidate are new faces, however.

Both Poroshenko and Tymoshenko played leading roles in the administrations that preceded Yanukovich's defeat of Tymoshenko in the 2010 election. Poroshenko, now a burly 48-year-old, later held a cabinet post for a time under Yanukovich.

Both became wealthy in the anarchic post-Soviet 1990s, Poroshenko, now worth $1.3 billion according to Forbes, through his candy and chocolate empire, Tymoshenko as the "gas princess" involved in the trade and transit of Russian natural gas.

After the Orange Revolution, when he was head of the National Security Council and she prime minister, the two traded accusations of corruption. Tymoshenko, 53, was jailed in 2011 for corrupt gas deals with Russia but was released when Yanukovich was toppled and her record cleared.

Olga, 82, strolling in central Kiev on Saturday, said Poroshenko was best placed to end six months of uncertainty.

"He is a businessman," she noted doubtfully as she made her way with her husband Nikolai across Independence Square, known as Maidan, where militant anti-Yanukovich protesters are still camped out, determined to hold new leaders to their promises.

"But he'll be good for the state and the people. He has factories near us and created jobs. He can calm things down."

But in the easternmost city of Luhansk, would-be voter Oleksander Cherednichenko doubted residents would be able to take part: "People are afraid that if they do go to a polling station that there will be gunmen there," he said.

"The best case scenario is that gunmen will just tell them to get out. The worst case scenario is that they shoot them." (Additional reporting by Sabina Zawadzki in Donetsk and Alastair Macdonald and Yvonne Bell in Kiev; Writing by Alastair Macdonald and Gareth Jones; Editing by Ron Popeski)

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